1. Field of the Invention
The present invention relates to a thermal printer and a resistance data measure device for a thermal head of the same. More particularly, the present invention relates to a thermal printer in which irregularity in performance of heating elements of a thermal head can be overcome in printing operation, and a resistance data measure device for a thermal head of the thermal printer.
2. Description Related to the Prior Art
There are widely used thermal printers including a thermal transfer printer in which ink film is used, and a direct thermal printer in which thermosensitive recording material is heated for directly printing an image.
The color thermal printer of the direct printing is used with a color thermosensitive recording material or recording sheet, in which thermosensitive coloring layers of magenta, yellow and cyan are formed on a support. Selectively to develop colors of the coloring layers, the coloring layers are different in the amount of heat energy (in mJ/mm.sup.2) to apply to. The lowest heat energy is required for coloring one of the coloring layers located at the obverse of the recording sheet. Higher heat energy is required for coloring the coloring layers according to the closeness to the support. Once a coloring layer is heated, electromagnetic rays are applied to it to fix it before heat energy for another coloring layer next to be colored is applied. This is for the purpose of destroying the further coloring ability of the present coloring layer to prevent it from being colored beyond desired density.
The thermal head includes an array of heating elements as resistors, which are arranged to record pixels arranged in one line. To record an image of each color thermally, heat energy is applied to the recording sheet as a sum of bias heat energy and image heat energy. The bias heat energy has an amount slightly short of causing the coloring layer to develop the one color, and is applied to the recording sheet during the bias heating at the beginning of recording each one pixel. The image heat energy has an amount determined according to the gradation level of one color, namely coloring density of the pixel to be printed, and is applied to the recording sheet during the image heating which succeeds the bias heating.
To reproduce high gradation, the heating operation is controlled finely. The heating elements of the thermal head need to have an equal resistance for the purpose of precise application of the heat as controlled. It is however inevitable that the heating elements have irregularity of 5-10% in the resistance. If the heating elements are driven for an equal duration, generated heat energy differs between the heating elements due to the differences in the resistance. Irregularity in density is likely to occur in an image being recorded.
U.S. Pat. No. 5,469,068 (corresponding to JP-A 6-79897) discloses a thermal printer in which the resistance of the heating elements is measured for the purpose of preventing occurrence of irregularity in printed density by compensating image data. The thermal printer is provided with a capacitor, of which capacitance is known. The capacitor is charged fully, and then discharged via the heating elements connected thereto. Time for decrease in a capacitor voltage is measured. For example, the decrease of the capacitor voltage down to a half of a power source voltage is checked to measure the discharging time. According to the discharging time and the capacitance of the capacitor, the resistance of the heating elements is calculated in view of a proportional relationship between the discharging time and the resistance of the heating elements.
This prior document also suggests use of a reference resistor to which the capacitor is connected, and of which resistance is known. The capacitor is charged fully, and then discharged via the reference resistor. The discharging time for decrease in the capacitor voltage is measured until coming down to a predetermined voltage. Again the capacitor is charged fully, and discharged via the heating elements.
The discharging time for decrease in the capacitor voltage is measured until coming down to the predetermined voltage. According to the resistance of the reference resistor and the discharging times via the reference resistor and the heating elements, the resistance of the heating elements is calculated.
According to U.S. Pat. No. 5,469,068 (corresponding to JP-A 6-79897), the capacitor is fully charged by applying voltage for a predetermined duration, and then discharged down from the capacitor voltage equal to the power source voltage, until the discharging time is measured. To measure the resistance of the heating elements with precision, the duration for the charging operation should be long enough for ensuring the full charging. A problem of the prior art lies in considerable slowness of measuring the resistance of all the heating elements. If the duration for the charging operation is shortened, the capacitor voltage upon the finish of the charging is not kept equal due to the charge having initially remained in the capacitor. Another problem lies in low precision in the measurement of the resistance.